1. Field of the Invention
The present invention relates to an organic electroluminescence device.
2. Description of the Related Art
Generally, an organic electroluminescence device (hereinafter, referred to simply as “organic EL device”) has a structure with an organic material laminate sandwiched between a transparent conductive film and a metallic electrode. The light (hereinafter, referred to as “EL light”) emitted within the device is extracted externally from the device through a transparent electrode.
The metallic electrode (backside electrode) arranged on a side where the EL light is to be extracted, which is the side opposite to a viewer, is made of a metal alloy such as an alloy of Mg and Ag or an alloy of Al and Li. The metal alloys having reflectance strongly reflects the external light which is incident from the outside. Therefore, the display thus obtained is largely influenced by the external light reflected at the backside electrode, and provides poor visibility like a mirror face when seen from a viewer.
In order to avoid such inconvenience, for the purpose of blocking the reflection of the external light, an expensive circularly-polarizing filter is generally attached to the exterior of the device. However, in a so-called “top emission structure” in which a light extracting electrode is arranged on an organic film but not on the side of a substrate, it is difficult to provide the circularly-polarizing filter to be in contact with the exterior of the device.
Even where the light extracting electrode is arranged on the side of the substrate, in a case where the substrate itself is made thin, the thickness of the circularly-polarizing filter hinders low-profiling and weight reduction. Further, the transmittance of the circularly-polarizing filter is about 40% so that the efficiency of the organic EL light is half or less the case with no filter.
Another known technique for preventing reduction in the visibility is to make the rear surface of the device non-reflective. A known method of making a non-reflective electrode is to use an absorbent material as an electrode. Japanese Patent No.2529741 discloses a method for manufacturing a device with low reflectance having a backside electrode in a laminated structure as shown in FIG. 4 using optical interference.
Still another known technique for preventing the reduction in the visibility is to make both electrodes transparent or semi-transparent and arrange a non-reflective film outside either electrode to suppress the reflection of external light at the backside electrode, thereby improving visibility.
The devices, in which the rear surface of the device is made low-reflective or non-reflective to improve visibility without providing a circularly-polarizing filter, provide an abrupt reduction in the efficiency of externally extracting light from the device.
The internally emitted light in the organic EL device is emitted with equal intensities in all the directions as shown in FIG. 1. Therefore, the light with the intensity equal to that emitted forward is also emitted in the rear direction. The light actually extracted outward from the device is the sum of electric field amplitudes of the light emitted forward and the light emitted in the rear direction and reflected by the backside electrode. Generally, in the organic EL device, the thickness of an optical film in a laminated device structure is optimized so that the phase difference between the forward emitted light and the reflected light of the rearward emitted light satisfies the condition of mutual intensifying of optical interference, thereby effectively extracting the light multiply reflected from the backside electrode and other reflecting planes as EL light emitted outward.
When the non-reflective electrode is used as the backside electrode, the EL light emitted to the rear side is not extracted on the front side. As a result, the intensity of the organic EL light thus externally extracted is supposedly about ½ of that of the entire EL light.
However, the actual experiment shows that the intensity of the organic EL light externally extracted has been reduced to about ¼ in the device equipped with the non-reflective electrode as compared with the ordinary EL device. This means that the efficiency is referred to about half of that of ordinary device equipped with the circularly-polarizing filter.